Placental mammals (
infraclassPlacentalia/plæsənˈteɪliə/) are one of the three extant subdivisions of the class
Mammalia, the other two being
Marsupialia. Placentalia contains the vast majority of extant mammals, which are partly distinguished from monotremes and marsupials in that the
fetus is carried in the
uterus of its mother to a relatively late stage of development. The name is something of a misnomer considering that marsupials also nourish their fetuses via a
placenta, though for a relatively briefer period, giving birth to less developed young which are then nurtured for a period inside the mother's
Placental mammals are anatomically distinguished from other mammals by:
a sufficiently wide opening at the bottom of the
pelvis to allow the birth of a large baby relative to the size of the mother.
the absence of
epipubic bones extending forward from the pelvis, which are found in all other mammals. (Their function in non-placental mammals is to stiffen the body during locomotion, but in placentals they would inhibit the expansion of the abdomen during pregnancy.)
retroposon presence/absence patterns has provided a rapid, unequivocal means for revealing the evolutionary history of organisms: this has resulted in a revision in the classification of placentals. There are now thought to be three major subdivisions or lineages of placental mammals:
Afrotheria, all of which diverged from common ancestors.
The living orders of placental mammals in the three groups are:
The exact relationships among these three lineages is currently a subject of debate, and four different hypotheses have been proposed with respect to which group is
basal or diverged first from other placentals. These hypotheses are
Atlantogenata (basal Boreoeutheria),
Epitheria (basal Xenarthra),
Exafroplacentalia (basal Afrotheria) and a hypothesis supporting a near simultaneous divergence. Estimates for the divergence times among these three placental groups range from 105 to 120 million years ago (MYA), depending on the type of DNA (e.g.
mitochondrial) and varying interpretations of
True placental mammals (the
crown group including all modern placentals) arose from stem-group members of the clade
Eutheria, which had existed since at least the
Middle Jurassic period, about 170 mya. These early eutherians were small, nocturnal insect eaters, with adaptations for life in trees.
True placentals may have originated in the
Late Cretaceous around 90 mya, but the earliest undisputed fossils are from the early
Paleocene, 66 mya, following the Cretaceous–Paleogene extinction event. The species Protungulatum donnae is sometimes placed as a stem-ungulate  known 1 meter above the
Cretaceous-Paleogene boundary in the geological stratum that marks the Cretaceous–Paleogene extinction event  and Purgatorius, sometimes considered a stem-primate, appears no more than 300,000 years after the K-Pg boundary; both species, however, are sometimes placed outside the crown placental group, but many newer studies place them back in
eutherians[further explanation needed]. The rapid appearance of placentals after the mass extinction at the end of the
Cretaceous suggests that the group had already originated and undergone an initial diversification in the Late Cretaceous, as suggested by
molecular clocks. The lineages leading to Xenarthra and Afrotheria probably originated around 90 mya, and Boreoeutheria underwent an initial diversification around 70-80 mya, producing the lineages that eventually would lead to modern primates, rodents,
However, modern members of the placental orders originated in the
Paleogene around 66 to 23 mya, following the Cretaceous–Paleogene extinction event. The evolution of crown orders such modern primates, rodents, and carnivores appears to be part of an adaptive radiation that took place as mammals quickly evolved to take advantage of ecological
niches that were left open when most dinosaurs and other animals disappeared following the
Chicxulub asteroid impact. As they occupied new niches, mammals rapidly increased in body size, and began to take over the large herbivore and large carnivore niches that had been left open by the decimation of the dinosaurs (and perhaps more relevantly competing synapsids). Mammals also exploited niches that the non-avian dinosaurs had never touched: for example,
bats evolved flight and echolocation, allowing them to be highly effective nocturnal, aerial insectivores; and whales first occupied freshwater lakes and rivers and then moved into the oceans. Primates, meanwhile, acquired specialized grasping hands and feet which allowed them to grasp branches, and large eyes with keener vision which allowed them to forage in the dark.
The evolution of land placentals followed different pathways on different continents since they cannot easily cross large bodies of water. An exception is smaller placentals such as rodents and primates, who left
Laurasia and colonized Africa and then South America via
A study on eutherian diversity suggests that placental diversity was constrained during the
multituberculate mammals diversified; afterwards, multituberculates decline and placentals explode in diversity.
^Amrine-Madsen, H.; Koepfli, K. P.; Wayne, R. K.; Springer, M. S. (2003). "A new phylogenetic marker, apoliprotein B, provides compelling evidence for eutherian relationships". Molecular Phylogenetics and Evolution. 28 (2): 225–240.
^Zoonomia Consortium (2020) A comparative genomics multitool for scientific discovery and conservation. Nature 587, 240–245
^O'Leary, Maureen A.; Bloch, Jonathan I.; Flynn, John J.; Gaudin, Timothy J.; Giallombardo, Andres; Giannini, Norberto P.; Goldberg, Suzann L.; Kraatz, Brian P.; Luo, Zhe-Xi; Meng, Jin; Ni, Michael J.; Novacek, Fernando A.; Perini, Zachary S.; Randall, Guillermo; Rougier, Eric J.; Sargis, Mary T.; Silcox, Nancy b.; Simmons, Micelle; Spaulding, Paul M.; Velazco, Marcelo; Weksler, John r.; Wible, Andrea L.; Cirranello, A. L. (8 February 2013). "The Placental Mammal Ancestor and the Post–K-Pg Radiation of Placentals". Science. 339 (6120): 662–667.
^Archibald, J.D., 1982. A study of Mammalia and geology across the Cretaceous-Tertiary boundary in Garfield County, Montana. University of California Publications in Geological Sciences 122, 286.
^Fox, R. C.; Scott, C. S. (2011). "A new, early Puercan (earliest Paleocene) species of Purgatorius (Plesiadapiformes, Primates) from Saskatchewan, Canada". Journal of Paleontology. 85 (3): 537–548.